import com.sun.tools.javac.Main;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class NQueens {

    List<List<String>> res = new ArrayList<>();

    public List<List<String>> solveNQueens(int n) {
        char[][] chessboard = new char[n][n];
        for (char[] c : chessboard) {
            Arrays.fill(c,'.');
        }
        backTracking(n,0,chessboard);
        return res;
    }

    public void backTracking(int n,int row,char[][] chessboard){
        if (row == n){
            res.add(Array2List(chessboard));
            return;
        }
        for (int col = 0; col < n; col++) {
            if (isValid(row,col,n,chessboard)){
                chessboard[row][col] = 'Q';
                backTracking(n,row+1,chessboard);
                chessboard[row][col] = '.';
            }
        }
    }
    public List<String> Array2List(char[][] chessboard){
        List<String> list = new ArrayList<>();
        for (char[] c : chessboard) {
            list.add(String.copyValueOf(c));
        }
        return list;
    }
    public boolean isValid(int row,int col,int n,char[][] chessboard){
        for (int i = 0; i < row; i++) {
            if (chessboard[i][col] == 'Q') return false;
        }
        for (int i = row - 1,j = col - 1; i >= 0 && j >= 0; i--,j--) {
            if (chessboard[i][j] == 'Q') return false;
        }
        for (int i = row - 1,j = col + 1; i >= 0 && j < n; i--,j++) {
            if (chessboard[i][j] == 'Q') return false;
        }
        return true;
    }

    public static void main(String[] args) {
        NQueens nQueens = new NQueens();

    }
}
